Sealing material

ABSTRACT

Sealing material comprises from 45% to 90% by weight exfoliated graphite, from 5% to 20% by weight thermosetting resin and from 5% to 50% by weight fibrous filler which is heat resistant at 250° C. The fibrous filler has fibres at least 90% of which have a fibre length of less than 200 μm, and an aspect ratio of less than 10:1.

The invention relates to a sealing material comprising exfoliatedgraphite, thermosetting resin, and a fibrous filler.

Sealing materials comprising exfoliated graphite have been used ascylinder head gaskets and other types of seals in internal combustionengines, such as automobile engines for some time, since they have goodheat resistance and stress relaxation properties. Graphite sheets havebeen found to have poor resistance to oil, and this has restricted theiruse. Means have been sought to improve the oil resistance of graphitesheets. Japanese patent application number 63-72780 relates to agraphite sheet which has expanded graphite particles, heat-resistantfibres, and an organic high-polymer binder as principal components. Thegraphite sheets produced are said to have improved oil resistance andantifreeze resistance. Heat-resistant inorganic fibres such as rockwool,ceramic fibres, silicate fibres and surface-treated silicate fibres orheat-resistant organic fibres such as aromatic polyamide fibres andphenolic resin fibres can be used as the heat-resistant fibres. Thelength of the heat-resistant fibres is said to be about 1-50 mm, and thethickness from 10-300 μm.

In use, when a sealing material comprising exfoliated graphite is underpressure, extrusion of the material may occur jeopardizing the seal.This occurs considerably more easily when the graphite is oil-soakedthan in an oil-free environment. Since such extrusion is an undesirablecharacteristic, it is desirable that extrusion of sealing material isavoided at pressures ordinarily experienced by them in oilyenvironments. The pressure at which extrusion occurs (the extrusioncollapse point) should be above such pressures.

According to the present invention, there is provided sealing materialcharacterised in that the material comprises from 45-90% by weightexfoliated graphite, from 5 to 20% by weight thermosetting resin, andfrom 5 to 50% by weight fibrous filler which is heat resistant at 250°C., the fibrous filler having fibres at least 90% of which have a fibrelength of less than 200 μm, and an aspect ratio of less than 10:1.

It has surprisingly been found that sealing materials comprising fibrousfillers with the short fibre lengths and aspect ratios of the presentinvention have considerably higher extrusion collapse points thansealing elements of graphite foil comprising exfoliated graphite alone,than graphite foil with phenolic resin, or than graphite foil comprisingphenolic resin and fibrous fillers having fibre lengths of similar sizeto those described in JP 63 72780. In some embodiments, the sealingmaterial is in the form of a sheet or layer, for example, of a planargasket such as an automotive head gasket.

In other embodiments the sealing material is in the form of a mouldedshape, such as a shaft sealing ring.

A sealing material according to the invention may comprise from 5 to 20%by weight of the thermosetting resin, and from 5 to 30% by weight of thefibrous filler.

Conveniently, the thermosetting resin is a phenolic resin.

When a filler is used in which at least 90% of the fibres of the fibrousfiller have a fibre length of less than 20 μm, a further increase in theextrusion collapse point is found, and so it is preferred to use fibresof this length. Use of a fibrous filler at least 90% of the fibres ofwhich have a fibre length of less than 10 μm is more preferable.

Advantageously, the aspect ratio of the fibrous filler is less than 6:1.

The exfoliated graphite is mixed with the fibrous filler (and,optionally, the powdered thermosetting resin) in the dry state, eg bygentle tumbling or in the airborne state. A layer of the mixture is thencompacted, usually by passage between rollers, to form a coherent foilor sheet. Alternatively, the mixture may be compacted to other shapes egsealing rings. Such other shapes may also be made by re-moulding foil.

When the resin is added as a free-flowing powder, it may subsequently bemade to flow, prior to cross-linking, by heating the consolidated foil(optionally under pressure). Further heating, normally to a highertemperature, then cross-linking the resin.

Alternatively, the powdered resin can be made to distribute itself moreeffectively through the foil by soaking in solvent and then drying.

A preferred met hod of introducing resin is to initially compress theexfoliated graphite to a relatively low density (eg 0. 5 kg m⁻³) so thatsome porosity is maintained. Liquid resin (solution or suspension inwater, etc) is then allowed to soak in. After drying, the low densityfoil is compressed further to achieve the required final density.

The graphite sheet preferably has a final density of from 0.7 to 1.5 kgm⁻³. Final densities of less than 0.7 are too weak and compressible.Densities of over 1.5 tend to be too hard and incompressible, giving apoor seal.

A particularly preferred fibrous filler is wollastonite the fibre lengthof which falls within the ranges of the present invention.

In addition to increasing the stress at which extrusion begins, shorterfibres and lower aspect ratios make it easier to mix the fibrous fillerwith the exfoliated graphite. This gives a more homogeneous product,which may contribute to improved performance.

A particular application of the sealing element of the present inventionis used in a multi-layer steel gasket in the form of a thin coating onthe gasket to fill fissures. Typically, such a sealing element will bein the range from 50 to 100 μm thick, preferably approximately 75 μmthick.

In another application of the sealing element, a graphite layer or sheetfrom 0.5 to 2 mm thick may be provided which acts to provide resiliencein a gasket.

COMPARATIVE EXAMPLE 1

Graphite foil having a thickness of approximately 200 μm was formed byconventional means from expanded graphite. The graphite foil wasconsolidated by passing through calenders to achieve a foil thickness of75 μm and a final density of 1.4 kg m⁻³.

COMPARATIVE EXAMPLE 2

Graphite foil having a final thickness of approximately 200 μm as formedin the first stage of comparative example 1 but with an intermediatedensity of 0.5 kg m⁻³ was impregnated with approximately 10% phenolicresin by first passing the foil through a bath containing a resolephenolic resin, Borden SC1008 resin in methyl isobutylketone solvent,and then by drying in an oven. The resin-impregnated graphite foil wasthen further consolidated as in comparative example 1.

COMPARATIVE EXAMPLE 3

Graphite foil comprising approximately 10% by weight of mica was formedas in comparative examples 1 and 2 except that 10% of mica was added tothe graphite prior to the expansion stage in the furnace. The foil wasthen impregnated and consolidated as in comparative example 2.

COMPARATIVE EXAMPLES 4 and 5

Comparative example 3 was repeated except that the graphite foilcomprised (in Comparative Example 4) 10% of Nygloss wollastonite havinga median fibre length of 0.25 mm or (in Comparative Example 5) FranklinFibre (calcium sulphate ex Franklin Institute with a length of about 1mm) instead of Mica. When it was attempted to make a graphite foil of 75μm thickness, the fibres coagulated and a foil was unable to be formed.The product was weak and inhomogeous and would not form a satisfactoryseal.

There now follow examples 1 to 6 which are illustrative of the presentinvention.

EXAMPLE 1

Comparative example 3 was repeated except that the graphite foilcomprised 10% of wollastonite instead of mica.

The wollastonite incorporated was Wollastocoat 10 (from Nyco minerals),having an aspect ratio of 3:1, a median fibre length of 3 μm, 96% of thefibres having a length below 10 μm.

EXAMPLE 2

Example 1 was repeated except that the wollastonite was Nyad 400 (fromNyco minerals) having an aspect ratio of 5:1 and a median fibre lengthof approximately 25 μm.

EXAMPLE 3

Example 1 was repeated except that the wollastonite was Vansil EW10(from Vanderbilt, UK distributor, Microfine Minerals), having an aspectratio of between 5:1 and 10:1, a median fibre length of approximately 32μm, 97% of the fibres having a length below 63 μm, and 40% below 20 μm.

EXAMPLE 4

Example 1 was repeated except that the wollastonite fibres formed 22% byweight and the final density was 1.1.

EXAMPLE 5

Example 1 was repeated except that the wollastonite fibres formed 24% byweight and the phenolic resin formed 18% by weight. The final densitywas 1.4.

EXAMPLE 6

Example 1 was repeated except that the wollastonite fibres formed 24% byweight and the phenolic resin formed 9% by weight. The final density was1.4.

Tests were carried out on the products of the foils produced in theexamples and the comparative examples as follows.

Each foil sample was soaked in a standard oil (ASTM oil 3) at 150° C.for 5 hours. The foils were then subjected to pressure, to find theirextrusion collapse points, that is the pressure at which extrusionoccurs after the foils have been soaked in oil.

The results were a s follows:

Extrusion collapse point (MPa) Comparative Example 1 85 ComparativeExample 2 87 Comparative Example 3 87 Comparative Examples 4 and 5 couldnot be determined Example 1 122 Example 2 99 Example 3 98 Example 4 164Example 5 186 Example 6 155

This shows that the inclusion of the wollastonite of short fibre lengthas in examples 1 to 6 significantly raises the extrusion collapse pointin this test.

What is claimed is:
 1. Sealing material wherein the material comprisesfrom 45-90% by weight exfoliated graphite, from 5 to 20% by weightthermosetting resin, and from 5 to 50% by weight fibrous filler which isheat resistant at 250° C., the fibrous filler having fibres at least 90%of which have a fibre length of less than 200 μm, and an aspect ratio ofless than 10:1.
 2. Sealing material according to claim 1, wherein thematerial comprises from 5 to 20% by weight of the thermosetting resin,and from 5 to 30% by weight of the fibrous filler.
 3. A sealing materialas claimed in claim 1, wherein the thermosetting resin is a phenolicresin.
 4. A sealing material as claimed in claim 1, wherein at least 90%of the fibres of the fibrous filler have a fibre length of less than 20μm.
 5. A sealing material as claimed in claim 4, wherein at least 90% ofthe fibres of the fibrous filler have a fibre length of less than 10 μm.6. A sealing material as claimed in claim 1, wherein the aspect ratio ofthe fibres of the fibrous filler is less than 6:1.
 7. A sealing materialas claimed in claim 1, wherein the fibrous filler is wollastonite.
 8. Asealing material as claimed in claim 1, wherein the sealing material isin the form of a sheet or layer.
 9. A sealing material as claimed inclaim 8, wherein the sheet or layer has a thickness of from 50 μm to 100μm.
 10. A sealing material as claimed in claim 1, wherein the sealingmaterial is in the form of a moulded shape.
 11. A sealing material asclaimed in claim 1, wherein the sealing material has a density of from0.7 to 1.5 kgm⁻³.